The
whistling thorn is an acacia tree commonly seen on
the savannas of equatorial East Africa,
particularly the Serengeti plain.

This acacia
can grow about 18 feet tall, but is often stunted
in its growth. The whistling thorn acacia protects
itself with pairs of long thorns up to 3 inches
long. Interspersed with these are modified thorns,
called stipular spines, which are joined at the
base by hollow bulbous swellings about 1 inch in
diameter. These are home to four different kinds of
stinging ants who pierce these swollen thorns with
tiny holes. When the wind blows it turns old and
abandoned spines into tiny whistling flutes, which
gives the tree its name.

It isn't
clear yet whether the relationship

with ants is a symbiotic or parasitic one. This particular
acacia doesn't have the toxic chemicals that ward off
insects and browsers like other species of acacias do. The
stinging ants protect the tree by swarming out of their
nests and attacking an intruder at the smallest movement.
Giraffes and other browsers are thought to be able to detect
the pheromones the ants give off, and leave the tree
alone.

However, some ant
species, like Crematogaster
nigriceps,
will prune the branches and flowers of their whistling thorn
so that enemy ant colonies on other trees can't get to their
tree. This pruning stimulates the acacia to produce a sugary
secretion at the ends of their leaflets which feeds the
ants. Unfortunately, it also kills the tree's growth tips
and effectively sterilizes it so it can't propogate
itself.

It is believed that
the ants have developed the habit of living in trees because
the soil of the savanna turns spongy in the rainy season and
dries out and cracks in the dry season. This makes it very
difficult for ant to build nests under ground.

The whistling thorn
acacia, like other acacias, has developed several ways to
survive the severely hot and dry climate in which it lives.
Because of the heat the tree must find ways to conserve
moisture. Their leaves have evolved into many tiny leaflets
(pinnae) which can turn to absorb sunlight, or avoid it and
reduce transpiration. The many leaflets are also beneficial
when animals graze on them. Some will be left behind to
continue the vital task of photosynthesis.

During the dry
season on the savanna, the whisling thorn acacia will drop
its leaves to conserve water. At the beginning of the rainy
season fragrant creamy-white flowers bloom before the leaves
grow back. The flowers look like little puffballs and
resemble those of the cultivated mimosa tree, which are in
the same family. Long seed pods develop, whose seeds are
very nutritious and a favorite of many animals, including
humans.